Need help diagnosing high coolant temperature issue

shbach · Dec 6, 2023

I've been having this issue with my 2008 Gen 2 Prius (240k miles) running hotter than it should and I was hoping the more experienced members here can help me figure it out.

Right now for short distances (< 30 minutes of driving) or just at speeds < 50 mph, the engine coolant stays around 190 F and the heater blows hot air. However, during extended drives on the freeway I've seen the coolant temperature climb up to as much as 237 F and the heater stops blowing hot air when this happens. Once I get down to lower speeds, the coolant temperature drops down to normal levels and the heater blows hot air again. The coolant temperature isn't getting hot enough during these instances to set off the alarm/warning lights in the car but I'm sure if this was during the summer or driving up a mountain it would hit that threshold.

Some history/background:

The first time I ever had overheating issues with my car was about 5 months ago. At that time the master warning light and high coolant temperature warning light were coming on. Once I changed the motor oil the issue went away.

About 4 months later my car started overheating (> 240 F) when driving short distances and the warning indicating high coolant temperature was displaying. I had a mechanic check/test for leaks but there were none found, bled the coolant system to remove potential air pockets, replaced the coolant heat storage tank (based on error code P1151), and yet the car still kept overheating.

Every single time the coolant system was bled, for a short while the car worked fine, heater blew hot air, and the coolant temperature stayed below 200 F even with extended drives on the freeway. Then for some reason (always in the afternoon or evening) the car would start overheating again (> 240 F), the heater no longer blew hot air, and the warning lights went off. Didn't know see how the time of day mattered but it could have been related to the ambient temperature. Anyways the overheating issue would persist until the system was bled again and at that point it worked fine again for a while.

More recently I had the thermostat replaced and now it is exhibiting the behavior I described at the beginning. It does appear to have been faulty because the car is overheating less now but the car is still running hotter than it should at times. Does anyone know what else could be causing this? Maybe the engine water pump is starting to fail?

Tombukt2 · Dec 6, 2023

That is interesting that sounds characteristic of some sort of air intrusion air bubble air pocket what have you. I had the coolant heat storage tank business same code some time ago and I think it's more the temp sensor that's near the storage or screwed into the storage tank more than the storage tank physically itself it's just some type of aluminum tank and as long as it's not hissing or leaking I would think there's predominantly nothing wrong with it I've had the code recently on a car I just got here and the cooling system is spot on everything is working The car seems to have heat fairly quickly no overheating but every so often I get the 11:51 so maybe I'll swap the sensor out of another tank and see if that changes anything but I get no air or anything so. It sounds like you have air getting into the system somehow 237 with the pressure cap on is quite high that should be on its way down I assume when you see this 237 fans are running what is the plastic coolant tank look like on top of the radiator is it maintained at least half full up to the crosshatches in the bottom or what have you? I would like to know because the generation 2 is usually not any kind of an overheater.

ChapmanF · Dec 6, 2023

In a Gen 2, the engine water pump is belt driven, so I wouldn't be thinking of any sort of intermittent water pump failure, as much as I might in the later generations with the electric pump.

In the past, whenever I have lost cabin heat in an overheating car, I've taken that to indicate a drastic loss of coolant and immediately pulled over to deal with it. But it's hard to explain the cabin heat coming back later that way. Coolant that's been lost doesn't magically reappear.

The Gen 2 has a coolant control valve that changes positions as commanded by the ECM, and depending on the position, it can take the cabin heater out of the loop. The ECM would not be sending it to that position on purpose during normal driving. But the valve has a position sensor that sometimes goes flaky. If the valve is in the right position but the sensor says it isn't. the ECM will try to move it till it is. So it might be useful to watch, on a scan tool, the position of this valve during the drive. But that would also be tricky, because what you'll see on the scan tool as the actual valve position will be what the sensor is saying, not necessarily the true position if the sensor is lying.

shbach · Dec 8, 2023

Tombukt2 said: ↑

That is interesting that sounds characteristic of some sort of air intrusion air bubble air pocket what have you. I had the coolant heat storage tank business same code some time ago and I think it's more the temp sensor that's near the storage or screwed into the storage tank more than the storage tank physically itself it's just some type of aluminum tank and as long as it's not hissing or leaking I would think there's predominantly nothing wrong with it I've had the code recently on a car I just got here and the cooling system is spot on everything is working The car seems to have heat fairly quickly no overheating but every so often I get the 11:51 so maybe I'll swap the sensor out of another tank and see if that changes anything but I get no air or anything so. It sounds like you have air getting into the system somehow 237 with the pressure cap on is quite high that should be on its way down I assume when you see this 237 fans are running what is the plastic coolant tank look like on top of the radiator is it maintained at least half full up to the crosshatches in the bottom or what have you? I would like to know because the generation 2 is usually not any kind of an overheater.
Click to expand...

So you think it's a sensor? I don't actually hear a fan running at 237 F. As far as I can tell there is no coolant loss in the system.

ChapmanF said: ↑

In a Gen 2, the engine water pump is belt driven, so I wouldn't be thinking of any sort of intermittent water pump failure, as much as I might in the later generations with the electric pump.

In the past, whenever I have lost cabin heat in an overheating car, I've taken that to indicate a drastic loss of coolant and immediately pulled over to deal with it. But it's hard to explain the cabin heat coming back later that way. Coolant that's been lost doesn't magically reappear.

The Gen 2 has a coolant control valve that changes positions as commanded by the ECM, and depending on the position, it can take the cabin heater out of the loop. The ECM would not be sending it to that position on purpose during normal driving. But the valve has a position sensor that sometimes goes flaky. If the valve is in the right position but the sensor says it isn't. the ECM will try to move it till it is. So it might be useful to watch, on a scan tool, the position of this valve during the drive. But that would also be tricky, because what you'll see on the scan tool as the actual valve position will be what the sensor is saying, not necessarily the true position if the sensor is lying.
Click to expand...

Do you think that control valve could be causing the loss of cabin heat? Could it also explain the too hot coolant temperature?

It is very strange. I've tested this multiple times now. The car runs fine with cabin heat and coolant at expected temperatures 190-200 F and doesn't go above that. After some time though if I keep driving for an extended time it's as if something can't keep up and the coolant temperature starts climbing past 215 F and the cabin heat is gone. If I stop the car for like 5 min things will go back to normal (coolant temperature goes back down to normal and cabin heat returns) and I can keep driving again.

Tombukt2 · Dec 8, 2023

Yeah the coolant control valve on these things is known to get iffy at high mileage usually they make noise and you can hear them clicking and running It's right there below the inverter pump but on the front cowl of the car right behind the radiator that whole support area there You can't miss it It's right there if you look next to your headlight adjuster You're staring at the water valve . Believe it or not I've never changed one I've got some cars here with over 300k and I've had them since the 150 mark or so so unless they were changed before that which is quite possible they look to be original and everything seems to be working fine and I don't fool with them until they don't I have cabin heat The car warms up quickly I'm definitely not overheating although I need to check the '09 the other night coming home with a refrigerator on the trailer not a big one I kept smelling that overheat smell that exhaust smell that where you're exhaust manifold is like running white hot I couldn't tell if it was me or another car no lights came on nothing so I'm not sure what was going on It certainly wasn't a cooling overheating issue would have been like an engine overheating because of over revving over driving who knows I'm guessing it just wasn't my car I was behind somebody that was burning themselves up.

shbach · Dec 28, 2023

A bit of an update on this issue: I took off the radiator cap and saw that that coolant level was low (didn't go up to the neck) so I filled up more coolant. After driving for a bit and checking again I saw there is coolant coming out from the radiator cap and there is a pressure build up. The coolant reservoir was over full and the coolant isn't being pulled back from there as the car cools. Also when taking off the radiator cap (when the car is cold) there is large release of pressure and coolant come out.

Does this pressure build up narrow it down to what it could be? Radiator cap, auxiliary water pump, coolant control valve, head gasket, or something else?

Thanks for the help!

Need help diagnosing high coolant temperature issue

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